---
title: "Random scan versus raster scan display"
subject: "Computer Graphics"
chapter: 1
key: "semester_5/computer_graphics:random-vs-raster-display"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/random-vs-raster-display
site: BCT Engineering
---

# Random scan versus raster scan display

Computer Graphics · Chapter 1: Introduction and Application

- Asked in: 2076 Chaitra, 6 marks; 2075 Asoj, 6 marks; 2073 Shrawan, 4 marks; 2072 Chaitra, 4 marks; 2071 Chaitra, 4 marks; 2069 Asar, 3 marks; 2068 Bhadra (old course), 5 marks (asked 7 times)
- Video: [Difference b/w Raster Scan & Random Scan Display | Computer Graphics](https://www.youtube.com/watch?v=mci4JeAq6b4) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/random-vs-raster-display
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-1

## Question

Differentiate between random scan (vector) and raster scan display technologies.

## Short answer

Random scan draws pictures by moving the electron beam directly along line endpoints from a display list, giving high resolution but poor shading. Raster scan sweeps the beam row by row, setting each pixel from a frame buffer, enabling realistic images at a fixed resolution with jaggies.

## Answer

A **random scan (vector) display** draws a picture as a set of straight line segments, moving the electron beam directly from one endpoint to the next. A **raster scan display** sweeps the beam over the whole screen row by row and turns each pixel on or off according to values stored in a frame buffer.

| Point | Random scan (vector) | Raster scan |
|---|---|---|
| Drawing method | Beam moves only along the lines of the picture | Beam sweeps every scan line, left to right, top to bottom |
| Picture storage | Display list (line-drawing commands) | Frame buffer (intensity of every pixel) |
| Memory needed | Small, depends on number of lines | Large, depends on resolution and bits per pixel |
| Resolution | Very high, smooth lines | Limited by number of pixels; lines show staircase (jaggies) |
| Refresh | Redrawn 30 to 60 times/s from display list; flicker if picture is complex | Fixed rate (e.g. 60 Hz) independent of picture complexity |
| Filled areas and colours | Poor at shading, solid areas and many colours | Good at filled regions, shading, photo-like images |
| Cost | Costly | Cheap, mass produced (TV technology) |
| Animation / editing | Easy to modify lines in display list | Needs the frame buffer updated |
| Examples | Pen plotters, early CAD displays | Monitors, TVs, LCD/LED screens |

```text
Random scan:  A ------> B
                        |
              D <------ C      (beam jumps along lines)

Raster scan:  ->->->->->->->|   (beam sweeps each row,
              ->->->->->->->|    horizontal retrace
              ->->->->->->->|    between rows)
```

Raster scan has replaced random scan in almost all present systems because it displays realistic, shaded and coloured scenes cheaply.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a random scan display draw a picture?
   A. By moving the beam directly from endpoint to endpoint along the lines only
   B. By sweeping every scan line from top to bottom
   C. By lighting each pixel stored in a frame buffer
   D. By refreshing a fixed 60 Hz grid regardless of lines
   Answer: A. Random scan moves the beam only along the line segments of the picture. (Key point: Drawing method)

2. What does a raster scan system store to hold the picture?
   A. A display list of line-drawing commands
   B. A frame buffer holding the intensity of every pixel
   C. Only the endpoints of the lines
   D. Equations of the geometric objects
   Answer: B. Raster systems keep pixel intensities in a frame buffer; vector systems keep a display list. (Key point: Picture storage)

3. Which statement about random scan is correct?
   A. It needs a large frame buffer and shows jaggies
   B. It is cheap and ideal for shaded photo-like images
   C. It needs little memory and gives smooth, high-resolution lines
   D. Its refresh time is independent of picture complexity
   Answer: C. Random scan stores only commands, so memory is small and lines are smooth. (Key point: Memory and resolution)

4. Why can a random scan display flicker?
   A. The beam sweeps too fast over every pixel
   B. The frame buffer is too small
   C. Horizontal retrace is too long
   D. The whole display list is redrawn 30-60 times/s, so a complex picture takes too long to redraw
   Answer: D. Refresh time depends on number of lines; a long list causes flicker, unlike raster whose refresh is fixed. (Key point: Refresh and complexity)

## Related in Computer Graphics

- [Short note: Raster scan display](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Ashort-note-raster-scan-display) — Computer Graphics, chapter 1
- [Raster versus vector graphics](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Araster-vs-vector-graphics) — Computer Graphics, chapter 1, asked 4×
- [Raster display and frame buffer load time](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Araster-system-load-time) — Computer Graphics, chapter 1
- [Explain vector display](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aexplain-vector-display) — Computer Graphics, chapter 1
- [Pixel access rate of raster system](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Apixels-per-second-access-time) — Computer Graphics, chapter 1

## Related in other subjects

- [Bitmap images; bitmap versus vector](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Abitmap-images-vs-vector) — Multimedia System, chapter 3